There really isn't an easy way to know. But, you just have to look for signs, such as how they act around you. Maybe if they get nervous around you, or if they stutter when they talk to you. There's a series of things that you can use to find out.
To know if your crush likes you when you are alone with her, observe to see if she will make any contact.
Ask him!
You should go for it if you know that person likes you go for it no holding back.
Ask Him.
No your ugly.
http://www.wikihow.com/Know-if-a-Guy-Likes-You
by the way your crush acts and looks around your friend
depends on if you know he likes you or not
If your crush's sister tells your sister that your crush likes you, it's a good sign that your crush may have feelings for you. It might be a good idea to talk to your crush directly to confirm these feelings and see where things go from there. Communication is key in situations like this.
If his old crush likes him still.....you should just wait until it's over cuz it would be kind of weird if you go out with him and his old crush is mad dogging you (staring at you with hate) But if his old crush doesn't then go for it. ok if the guy is good looking you should go for it but if his old crush is pretty mad tell him the truth that u should be just frineds until his old crush is over him because you don't wanna make anyone mad hope this helps:)
confront him.
I dont think so but if you find out let me know!